As of now, the largest company (in terms of users) that offered an API that provided “check-in” data (although in Latitude you don’t need to check-in) was probably Twitter or maybe Yelp!, followed by Brightkite and Foursquare. Neither Microsoft, Yahoo! or Facebook (yet) even have check-in apps/services, nevermind offer an API for them, so Google is the only mega-company that has really taken the leap into this space.

Make no mistake, it’s a leap – not of faith, but of privacy concerns – and Google makes it very clear in their announcement of the API that users will have fine-grained privacy controls of the Latitude data, including app-by-app controls. Of course, most apps these days kind of force you to either share all your available data – you’re only real option in most cases being to simply not install the app.

Regardless, the real question is: will developers spend time building either dedicated Latitude apps or integrating the API into their existing applications? Our guess is the latter, even though Latitude claims to have 3 million users, it’s a commonly held belief that engagement with other check-in services is much higher. So we expect that current apps like Tweetdeck, Check.in, Google Maps mashups and check-in aggregators such as Checkinmania will add the API, but dedicated Latitude apps will be sparse. Also, count tracking apps such as TrackU among those that we’d expect to be very happy that this API is out.

One final interesting note from the blog post, is that Google makes a mention about the always-on aspect of Latitude in regards to battery life especially:

“We’ve also learned that making your phone’s continuous location available in the background is tricky to do accurately and efficiently — just imagine your phone’s battery life if several apps were continuously getting your location in different ways? With this in mind, we built a free and open Latitude API that lets the third-party developers you choose start using your updated location in new ways without reinventing the wheel.”

This is kind of a vague statement that makes it sound like developers will have the option to not use always-on Latitude data, perhaps, for example just checking in every hour (developers could in turn use this battery-saving as incentive to get users to use their apps over others).

The API itself, as Lee pointed out to us in March, is a pretty basic REST API. The two screenshots from the developer reference guide show what you can basically do with the API (click images to see larger view):